A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus may include a positioning system to position a moveable object, such as a substage with respect to a reference object, e.g. a metrology frame or a main frame. The substage may be or include a support structure to hold the substrate or patterning device.
Usually, the positioning system includes a main stage which is moveable with respect to a frame in a moving direction, wherein the substage is moveable in the moving direction between a first and a second position relative to the main stage. The substage may be supported by the main stage. Generally, a long stroke actuator is provided between the frame and the main stage as a first actuator to apply a force to the main stage with respect to the frame in the moving direction, and a short stroke actuator is provided between the main stage and the substage as a second actuator to apply a force to the substage with respect to the main stage in the moving direction. The long stroke actuator is used for coarse positioning of the main stage and substage, and the short stroke actuator is used for fine positioning of the substage relative to the main stage. The short stroke actuator may be a moving magnet system in which the stator, i.e. a part of the main stage, includes a coil system which is surrounded by a mover, i.e. a part of the substage, which includes magnets incorporated in magnetically highly permeable metal in order to achieve high magnetic field densities.
With this configuration, when the long stroke actuator accelerates the main stage with a certain acceleration, the short stroke actuator also has to apply a force between the substage and the main stage to accelerate the substage with the same amount. The force to accelerate the substage is in fact generated twice, once by the long stroke actuator and once by the short stroke actuator. As a consequence, the short stroke actuator is designed in accordance with these demands, resulting in a relatively large and heavy short stroke actuator and thus a relatively high mass of the substage, so that an even larger force has to be generated to accelerate the substage. Also, when the substage is accelerated with a certain amount, a lot of heat is generated by the short stroke actuator in or near the substage which causes structural deformations resulting in e.g. a loss of position measurement accuracy and deformation of the patterning device or substrate on top of the substage.
To cool the substage, hoses carrying a cooling fluid may be provided between the substage and the main stage or frame, thereby introducing force disturbances that limit the position accuracy of the positioning system.